1. Field of the Invention
This invention relates to a novel CATV signal distribution system having coaxial lines and signal amplifiers therein. The invention particularly relates to local means for indicating when the output of one or more of the amplifiers is outside its prescribed operating range.
2. Description of the Prior Art
A CATV signal distribution system collects television and other electronic communications signals from one or more of various sources, such as large antennas for off-the-air transmissions, by a satellite receiving station, or directly from locally generating sources by telephone line, cable, microwave, etc. These raw signals are collected at a head end where they are processed to put them in a form for distribution to subscribers. Once processed, the signals are combined and routed to the subscribers through trunk and feeder paths of the system.
Each subscriber connects to a tap in a local feeder line which provides signals in a multiplicity of channels, which are generally six (6) MHz in bandwidth. A 300 MHz system has a capability of about thirty-five (35) channels. Also, many CATV signal distribution systems can also transport signals "upstream"; that is, from the subscriber to the head end or to an intermediate point in the system. There are many other variations on the basic form of the system described above. More detailed information on CATV signal distribution systems may be found in Cable Communication by T. F. Baldwin et al. (Prentice-Hall, Inc., Englewood Cliffs, N.J. 1983) and Cable Television by W. Grant (Reston Publishing Co., Inc., Reston, Va. 1983).
In a typical CATV signal distribution system, the combined signals at the head end are fed into a trunk line where they pass over some distance to a splitter, which divides the combined signals and passes them to two (2) or more trunks. The signals then pass in each trunk line to feeder lines, each of which is provided with a plurality of taps. The trunk and feeder lines are usually coaxial cables which are strung on poles or are underground in conduits.
As the combined signals travel downstream in the coaxial cables to the subscribers, loss is incurred and the power level becomes progressively lower, and may fall below prescribed operating parameters required for acceptable subscriber use. Low signal amplitude may result in a snowy picture on the subscribers' television set.
To remedy this problem, the CATV system includes wide-band amplifiers at each junction with the trunk and feeder lines and, optionally, at intermediate points in the lines. Such amplifiers compensate for the drop in power level due to losses in the system and due to the splitting of the signals. Each amplifier is adjusted to provide an output within a prescribed range of operating parameters. If the amplifier is out of adjustment, the output may be too strong, in which case the subscriber may see wide jumpy bands in the picture on his/her television set. Too weak an output signal may result in a snowy picture, as mentioned above.
When the subscriber telephones the system operator and complains about the quality of the signals at his/her tap, a service person must go to each amplifier that is suspect of malfunctioning, or of being out of adjustment, and test the output at the test jack in each amplifier. This is a time consuming and strenuous job, since it requires climbing poles for polehung coaxial lines and descending underground for coaxial lines in conduits in order to access each amplifier.
Some prior systems provide status monitoring electronically where there is the capability of passing test signals upstream in the system. Through sequential polling of each amplifier in the system, data for each amplifier can be displayed to identify present and/or potential amplifier problems. While this monitoring process may reduce fault-finding time and cost by remotely identifying problem areas, a service person must, nevertheless, go to and test each suspect amplifier. Such prior systems require the amplifier location and the malfunction to be coded. Such coding required additional signal processing equipment at the amplifiers and at the head end, thereby adding substantial additional cost and maintenance to the system.